There are today several characteristic types of bath and associated process conditions for phosphating metal surfaces. Conventional zinc phosphate baths work at comparatively high temperatures in the range of from 50.degree. to 60.degree. C. and form substantially iron-free zinc phosphate layers on the metal surface. Subsequent developments have made it possible to incorporate iron in the zinc phosphate layers deposited and, hence, to produce particularly satisfactory, stable zinc phosphate layers using baths comparatively poor in zinc and rich in phosphate, again at comparatively high temperatures. Thus, U.S. Pat. No. 4,265,677 describes aqueous, acidic phosphating solutions having a ratio by weight of Zn to PO.sub.4 of 1:(12-110) for the surface treatment of metals. The thin and uniform phosphate coatings are particularly suitable as a base for subsequent electro-dip-lacquering. Known accelerators for phosphating baths of the type in question are, for example, nitrite ions and/or aromatic nitro compounds, cf. U.S. Pat. No. 4,292,096.
By contrast, British patent No. 2,093,075 A seeks to obtain better results by working at temperatures in the range of from 30 .degree. to 60.degree. C. with chlorate-containing zinc phosphate solutions containing from 0.5 to 1.5 g/l of Zn, from 0.4 to 1.3 g/l of Ni, from 10 to 26 g/l of P.sub.2 O.sub.5 and from 0.8 to 5 g/l of ClO.sub.3, to which no nitrite is added and in which the ratio by weight of Zn to Ni is adjusted to a value of 1:(0.5-1.5), the ratio by weight of Zn to P.sub.2 O.sub.5 to a value of 1:(8-85) and the ratio of free P.sub.2 O.sub.5 to total P.sub.2 O.sub.5 to a value of 0.005 (at approximately 30.degree. C.)-0.06 (at approximately 60.degree. C.):1. The quality of the phosphate layers obtained by this process is said to be critically determined by maintenance of the concentration ratio between Zn and P.sub.2 O.sub.5. For a P.sub.2 O.sub.5 .fwdarw.PO.sub.4 conversion factor of 1.338, the lower limit of that ratio (Zn/P.sub.2 O.sub.5 =1/8) is at 10.7 parts by weight of PO.sub.4 to 1 part by weight of Zn.
All these proposals of the prior art use comparatively high total acid contents or, in other words, are characterized by a high consumption of chemicals per unit volume of the aqueous treatment solution. Any reduction in this consumption of chemicals would improve the economy of phosphating processes of the type in question to a very considerable extent. For example, the loss of material attributable to carryover would be distinctly reduced, which would in turn reduce the overall consumption of chemicals.